Unless you find a used one, you know where, $100 is about right. Actually, there are several different types needed (takeup torque, back torque, Fast mode torque, etc...$100 a pop). I can justify it for myself, considering I specialize in cassette deck restoration. Even so it is last on the list of equipment to acquire.
For someone who wants to monitor performance of his own deck, you will need to determine worth for yourself.
The best ones available come direct from Teac, but be prepared to wait awile to get them (2 or 3 months, maybe more, since it is a special order item). Teay are my source for calibration tapes as well...
Enjoy,
Rich P